You are looking at Sea Surface Temperature anomalies in degrees
Centigrade, or "SST anomalies" for short. Yellows and purples
mean warm, greens and blues mean cold. SST anomalies are how much temperatures
depart from what is normal for that time of year. This makes sense; we
might say that we had a "warm winter" even though it was still
much colder than summer. What we mean is that it was warmer than a normal
winter; in our parlance, we would say that it was a "positive anomaly".
An unusually cold winter would be a "negative anomaly". For Pacific
SST, an anomaly in the range of 1.5 to 3.5 degrees Centigrade would be
considered characteristic of an El Nino; the warmer and more widespread
the water, the stronger the El Nino.